Rotational molding process for producing foamed articles

ABSTRACT

Improved compositions useful for the production of rotomolded articles having a foamed interior and non-foamed exterior skin are provided. The compositions of the invention are comprised of a first thermoplastic resin component which is an ethylene polymer in pellet form containing a chemical foaming agent, an organic peroxide and, optionally, a metal-containing activator compound and a second resin component which is a powder and can be a thermoplastic ethylene polymer or ethylene copolymer having less than 30% crystallinity. An improved one-step process for producing foamed rotomolded articles having foamed interiors and non-foamed exterior skins is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 09/114,977,filed Jul. 14, 1998, now U.S. Pat. No. 5,992,778, which is acontinuation-in-part of U.S. application Ser. No. 08/842,777, filed Apr.17, 1997, now U.S. Pat. No. 5,783,611, which claims the benefit of U.S.Provisional application Ser. No. 60/018,261 filed May 24, 1996.

FIELD OF THE INVENTION

This invention relates to improved compositions useful for producingrotationally molded articles having foamed interiors and non-foamedexterior skins and to the process of producing such articles.

BACKGROUND OF THE INVENTION

Rotational molding, also referred to as rotomolding, is widely used toproduce hollow articles such as toys, sporting equipment, containers,water tanks, etc. For the process, a thermoplastic resin is placed in amold which is then closed, heated and rotated on two axes, i.e.,biaxially, to allow the resin to melt and uniformly coat the interior ofthe mold. The mold is then cooled and the molded article is removed.

In many instances, it is highly desirable to have a foam layer or corein the interior of the molded article to provide insulation, impartstructural integrity or stiffness to the article, reduce weight, or thelike. This is accomplished by including a foaming or blowing agent withthe resin which decomposes at the molding temperature to release a gas,such as CO₂ or N₂.

The use of foaming agents presents a problem where formed articleshaving a smooth exterior surface are desired and various techniques havebeen employed to produce foamed rotomolded goods having a smooth skinlayer. In one approach, referred to as the "two-step" method, anon-foamable resin is first introduced into the mold and molded toproduce a non-foamed exterior layer of the desired thickness. A foamableresin is then introduced into the mold and the molding operation resumedso that a foamed layer is formed on the inside of the non-foamed layer.Such a method is disclosed in U.S. Pat. No. 3,976,821. While it ispossible to produce acceptable molded goods in this manner by properselection of resin(s) and operating conditions, the procedure is laborintensive and time-consuming. Also, it requires use of a mold with anopening to permit introduction of the foamable resin.

Another approach has been to utilize specially designed equipment, suchas disclosed in U.S. Pat. No. 4,952,350, which permits both thenon-foamable and foamable resin to be introduced at the beginning of theoperation but maintained separately. In this way, the foamable resin canbe released, e.g., from a dump box, at some point in the operation afterthe non-foamable resin has melted and uniformly coated the interiorsurface of the mold.

In other cases, such as in U.S. Pat. No. 2,989,783, the foamable resinis enclosed in a thermoplastic bag. The foaming or release of gas fromthe foamable resin forces the bag to expand to the shape of the mold sothat the bag forms the outer surface of the article.

Still other approaches have been suggested to produce acceptablerotomolded goods having a smooth skin layer and foamed inner layer in a"one-step" process. These procedures include, for example, processeswhich rely on density differentials of the foamable and non-foamableresins. However, since density differences of most of the commonly usedthermoplastic resins are slight, it is not possible to achieve sharpseparation of the foamable and non-foamable layers using this approach.

Another method disclosed in U.S. Pat. No. 3,962,390 relies on thedifferent heat capacities of the foamable and non-foamable resins. Byusing a resin having a greater heat capacity for the foamable resin,particles of the resin with the lower heat capacity begin to melt firstand thereby coat the inner surface of the mold before the resin with thefoam agent begins to melt.

While all of the above methods are capable of producing foamed articleswith an exterior surface of reasonable quality under optimum processingconditions, with certain resins they are still prone to surface pitting,i.e., the presence of surface pores or pinholes. Surface pores, whensufficiently large and/or present in large numbers can severely detractfrom the appearance of the molded good and render the articleunacceptable. Also, in other instances where the non-foamed skin resinis colored and the foamable interior resin particles is uncolored,surface blotches or blemishes are evident as a result of the interiorfoamed resin "pushing through" the skin layer. This is particularly truewhere a thin skin layer is desired and where, as is most usually thecase, the foamable resin is in the form of pellets. Obviously, thislatter problem could be overcome by coloring the foamable resin but thiswould increase the cost.

These and other disadvantages associated with heretofore known processare overcome by the composition and process of the present inventionwhereby it is possible to produce foamed rotomolded articles having anexterior skin which is substantially free of surface defects, such aspitting and color blotches, and a foamed interior.

SUMMARY OF THE INVENTION

In accordance with the present invention, in a first highly usefulembodiment there is provided improved compositions useful for theproduction of rotomolded articles having foamed interiors and non-foamedexteriors comprising: (a) 25 to 75 percent by weight, based on theweight of the total composition, ethylene polymer pellets ranging insize from 1/16 inch to 3/16 inch in diameter and containing 0.25 to 7.5weight percent chemical foaming agent, based on the weight of theethylene polymer, said ethylene polymer having a melt index from 0.25 upto 25, and (b) 75 to 25 percent by weight, based on the weight of thetotal composition, ethylene polymer powder mixture containing: (i) amajor proportion of fractional melt index ethylene polymer powderwherein 80 percent or more of the powder particles are greater than 250microns in size and (ii) a minor proportion of ethylene polymer powderwherein 80 percent or more of the, powder particles are less than 250microns in size and the ethylene polymer has a melt index greater than 1g/10 mins. The mean particle size of the two powder components, i.e.(b)(i) and (b)(ii) differ by at least 100 microns. The powder mixturemost commonly consists of 75 to 97.5 percent (b)(i) preferably having aparticle size from 250 to 1000 microns and 2.5 to 25 percent (b)(ii)preferably having a particle size from 10 to 250 microns.Azdocarbonamides and modified azodicarbonamnides are the preferredfoaming agents. Particularly useful results are obtained when (a) ishigh density polyethylene, (b)(i) is low density polyethylene and(b)(ii) is linear low density polyethylene.

In another useful embodiment of the invention up to 2.5 weight percentorganic peroxide and, optionally, up to 4 weight percentmetal-containing activator compound, are included with the chemicalfoaming agent in the ethylene polymer pellets. The inclusion of anorganic peroxide with the foaming agent enables the processor to reducethe density of the foam and thereby reduce the overall weight of themolded article and decrease the cycle time. This also broadens theprocessing window by increasing the thermal stability of the foam.

In yet another highly useful embodiment, flexible foamed rotomoldedarticles are produced using compositions wherein one or more ethylenepolymers having rubber-like characteristics; such as ethylene-vinylacetate copolymers, ethylene-alkyl acrylate copolymers andethylene-α-olefin copolymers, are employed. For these formulations, thechemical foaming agent may constitute up to 20 weight percent of thepellet component along with up to 2.5 weight percent organic peroxideand up to 4 weight percent metal-containing activator compound. Thepowder component may also be comprised, in whole or in part, of arubbery ethylene copolymer. Compositions which utilize foamable LDPEpellets and EVA copolymer powders are particularly useful for thisembodiment.

An improved process for utilizing said compositions wherein said polymerpellets containing the chemical foaming agent and optional organicperoxide and activator and said polymer powders are combined and heatedin a mold at a temperature above the melt temperature of the polymersand above the decomposition temperature of the foaming agent whilebiaxially rotating the mold to produce foamed molded articles havingsurfaces which are substantially free of surface defects, such aspinholes and color blotches, and a foamed interior is also provided.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to foamable polyolefin resin compositionsand to foamed rotomolded articles produced therefrom having a non-foamedexterior skin. The articles may be hollow or the interior can becompletely foam filled. The hollow articles may be flexible or have arigid or semi-rigid structure depending on the resins employed, therelative thickness of the foamed and non-foamed layers, and the amountof chemical foaming agent and optional additives used.

The compositions of the invention are comprised of two distinctthermoplastic resin components, namely, a first component which is anethylene polymer in pellet form containing the chemical foaming agentand any additional additives, such as peroxide and activator, and asecond powder component which is a mixture of different particle sizeand melt index ethylene polymer powders.

Depending on the particular application involved, i.e., the rotomoldedarticles being produced, the weight ratio of pellet to powder may bevaried within wide limits. In general, however, the first resincomponent, also referred to herein as the foamable resin or pelletizedresin, constitutes from 25 to 75 weight percent of the total compositionand the second powder resin component constitutes from 75 to 25 weightpercent of the total composition. More preferably for certainapplications, the pelletized resin is present in an amount from 30 to 60weight percent and the resin powder is present in an amount from 70 to40 weight percent.

While any of the well-known polyolefin rotomolding resins can be usedfor the compositions of the invention, polyethylene resins are mostgenerally employed to obtain the first (pellet) component and second(powder) component. As used herein the term "polyethylene" encompassesboth homopolymers of ethylene and copolymers of ethylene with C₃₋₈α-olefins, vinyl C₂₋₄ carboxylates, C₁₋₄ alkyl (meth)acrylates,typically having ethylene as the predominant monomer. Commonly usedpolyethylenes include very low density polyethylene (VLDPE), low densitypolyethylene (LDPE), linear low density polyethylene (LLDPE), mediumdensity of polyethylene (MDPE), high density polyethylene (HDPE) andvery high density or ultra high molecular weight polyethylenes, producedusing well-known Ziegler. Phillips or metallocene polymerizationcatalysts and procedures.

Ethylene-vinyl carboxylate and ethylene-alkyl (meth)acrylate copolymerscan also be utilized for the compositions of the invention and are alsoencompassed by the term "polyethylene" as used herein. These ethylenecopolymers can have a rubber-like characteristic and are particularlyadvantageous for the production of flexible foamed rotomolded articles.As employed herein the terms "rubbery" and "rubber-like" refer topolymers that can be stretched at room temperature to at least twicetheir original length and after having been stretched and the stressremoved, return to approximately their original length in a relativelyshort period of time. Such polymers will typically have less than about30% crystallinity and, more commonly, less than 20% crystallinity asdetermined by measuring the area of the crystalline melt peak bydifferential scanning calorimetry (DSC) in accordance with knownprocedures.

Useful rubbery ethylene copolymers include ethylene-vinyl acetatecopolymers, ethylene-alkyl acrylate copolymers and ethylene-α-olefincopolymers. Particularly useful ethylene-vinyl acetate copolymers willgenerally contain from 8 to 50% vinyl acetate and more preferably, 10 to45% vinyl acetate. The ethylene-alkyl acrylate copolymers will generallyhave from 1 to 50% and, more preferably, 5 to 50% alkyl acrylate, suchas ethyl acrylate or n-butyl acrylate, copolymerized with the ethylene.Ethylene-α-olefin copolymers can contain from about 70 to 80% α-olefinhaving from about 3 to 8 carbon atoms, most preferably, propylene,butene-1, hexene-1 and mixtures thereof. When the alpha-olefin ispropylene, it will typically be present in an amount from about 20 to80% whereas the higher alpha-olefins are generally employed in amountsfrom 7 to 40%. The foamable portion of the rotomolding composition,i.e., first component, is comprised of a pelletized ethylene polymerhaving a melt index from 0.25 g/10 mins. up to about 25 g/10 mins. whichcontains the chemical foaming agent and, optionally, other additivessuch as peroxide, foam activators and the like. The pellets range insize from about 1/16 inch to about 3/16 inch in diameter. In one usefulembodiment, the ethylene polymer has a melt index from 1 to 10 g/10mins. The chemical foaming or blowing agent will comprise up to 20%weight percent, based on the weight of the ethylene polymer and, mostcommonly, will range from 0.25 to 20 weight percent. For certainapplications where the compositions are to be used to produce rigidfoamed articles, the foaming agent will be present from 0.25 to 7.5 and,more preferably, from 0.5 to 5 weight percent. With formulations whereorganic peroxide is used and particularly where flexible foamed articlesare to be produced, the foaming agent will generally be employed in anamount from about 2.5 up to about 18 weight percent and, morepreferably, from about to 10 weight percent.

It is particularly advantageous, when rotomolding rigid foamed articles,if the ethylene polymer is a high density polyethylene. HDPEs havedensities in the range 0.941 g/cm³ to 0.970 g/cm³ and impart stiffnessto the foamed interior layer or core of the rotomolded article. Also,HDPE has a higher melt temperature than lower density ethylene polymerswhich is desirable for optimal results in these applications. UsefulHDPE polymers include homopolymers of densities 0.960-0.970 g/cm³ andcopolymers, usually with butene-1 or hexene-1, of densities 0.941-0.959g/cm³.

Densities reported herein for the ethylene polymers are determined inaccordance with ASTM D-1505. Melt indexes referred to herein aredetermined in accordance with ASTM D-1238-57T at 2160 grams load and190° C. Melt indexes are reported in g/10 mins.

Conventional chemical foaming agents are employed with the ethylenepolymer and are incorporated utilizing known procedures. Typically, thepolyethylene, chemical foaming agent and any optional ingredients aremixed in an extruder at a temperature above the melt temperature of theresin but below the decomposition temperature of the chemical blowingagent and organic peroxide if used. The melt is then passed throughsuitable die, such as used with a pelletizer, to obtain the pelletizedresin.

The foaming agents can be any of the known organic or inorganiccompounds or systems which decompose at elevated temperatures to releasea gas such as N₂ or CO₂. Organic foaming agents, sometimes also referredto as blowing agents, include azodicarbonamide (ADCA) and modifiedazodicarbonamide, i.e., ADCA modified with zinc oxide, calcium carbonateor the like to lower the decomposition temperature and activate thesystem, 5-phenyltetrazole, dinitrosopentamethylene tetramine,azobisisobutyronitrile, diazoaminobenzene,oxybis(benzenesulfonylhydrazide) and the like. Inorganic foaming agentscan include sodium borohydride, ammonium carbonate, sodium bicarbonateand modified sodium bicarbonate, i.e., sodium bicarbonate modified witha proton donor such as citric acid, and the like. ADCA, modified ADCA,sodium bicarbonate and sodium bicarbonate/citric acid foaming agents aremost commonly used for the compositions and process of this invention.ADCA and modified/activataed ADCA are particularly useful incompositions which contain an organic peroxide.

While it is not necessary, additives which function to control or modifyfoam cell size or foam density or modify/control the temperature or rateof decomposition of the chemical blowing agent may also be included withthe ethylene polymer. Useful additives of this type, generally referredto herein as metal-containing activators, include calcium carbonate,titanium dioxide, zinc oxide, zinc stearate, calcium stearate, dibasiclead phthalate, dibasic lead stearate, chromium in the form of trivalentor hexavalent cations, and the like. When present, the amount of theseactivators can range up to about 4 percent by weight, based on theweight of the ethylene polymer. Most typically, the amount of activatorused is between 0.01 and 2.5 weight percent.

To achieve the desired improvements with the composition and process ofthe invention, a second component referred to herein as the non-foamablecomponent or powder component is necessarily used with the pelletizedresin component containing the foaming agent described above. Theethylene polymer(s) used for the powder component will be the same asused to prepare the first (pelletized) component containing the foamingagent or they can be different. In one embodiment of the inventionparticularly useful for producing rigid rotomolded goods having a toughexterior skin, a powder mixture comprised of two distinct ethylenepolymer powders is employed. The two powders used for the secondcomponent may be derived from the same type of ethylene polymer providedthat the ethylene polymer powders have different particle sizes anddifferent melt indexes. The principal or primary powder component whichcomprises the major portion, i.e., greater than 50 percent, of thepowder mixture is an ethylene polymer powder wherein 80 percent or moreof the powder particles are 250 microns in size and wherein the ethylenepolymer has a fractional melt index. As used herein, the term"fractional" melt index refers to melt indexes less than 1 g/10 mins Theminor powder component which constitutes less than 50 percent of thepowder mixture is an ethylene polymer powder wherein 80 percent or moreof the powder particles are less than 250 microns in size and whereinthe ethylene polymer has a melt index greater than 1 g/10 mins. The meanparticle size of the two powder components should differ by at least 100microns. More typically, the mean particle size of the powder componentswhich comprise the mixture will differ by 150 microns or more.

The particle size of the major powder component most generally rangesfrom 250 to 1000 microns and, more preferably, these powder particlesrange from 250 to 600 microns in size. The particle size of the minorpowder component most generally ranges from 10 to 250 microns and, morepreferably, these powder particles range from 20 to 225 microns in size.It will be understood for the above particle size ranges, as well as forother particle size ranges referred to herein, that 80 percent or moreof the particles will fall within the specified size limits. Particlesizes are determined using standard screening, i.e., sieving procedures.

The larger particle size powder component most generally constitutesfrom about 75 to about 97.5 percent of the total powder mixture with thesmaller particle size powder constituting from 2.5 to 25 weight percent.More preferably, the larger particle polymer comprise 85 to 95 weightpercent of the powder mixture with the smaller particle size polymercomprising the balance.

In one highly useful embodiment of this invention the ethylene polymercomprising the major powder component and from which the larger sizepowder particles are derived has a melt index from 0.1 to 0.8 g/10 mins.and, more preferably, from 0.2 to 0.5 g/10 mins and is LDPE having adensity from 0.915 to 0.930 g/cm³. The ethylene polymer which comprisesthe minor powder component and from which the smaller particle sizepowder particles are formed preferably has a melt index from 1 to 10g/10 mins. and, most preferably, from 2 to 7 g/10 mins and is LLDPEhaving a density from 0.915 to 0.940 g/cm³. LLDPEs are conventionallyobtained by polymerizing ethylene with butene-1, hexene-1 or octene-1.LDPEs are conventionally obtained by the high pressurehomopolymerization of ethylene and characterized by having long chainbranches which are formed during the polymerization.

It is, however, also possible to utilize low density copolymers ofethylene with monomers which contain polar groups, such as vinylacetate, ethyl acrylate, n-butyl acrylate or the like, as all or aportion of the powder component. Utilizing copolymers of the latter typewhich have rubbery characteristics makes it possible to produce flexiblefoamed rotomolded articles. When utilizing a rubbery copolymer for thepowder component, it may be present by itself or used with any of theaforementioned ethylene polymers useful for this purpose. Whenrotomolding flexible foam-filled products, if more than one resin isused for the powder component, it is not essential that the differentresin powders be of different particle size or particle sizedistribution.

In addition to the organic peroxide and metal-containing activatorcompound which can be included in the foamable ethylene polymer resincomponent, other additives may also be present in small amounts witheither the foamable and/or non-foamable components. Such additives mayinclude pigments and colorants, UV stabilizers, antioxidants,anti-static agents and the like. Typically, when present, theseadditives are used in amounts less than 1 percent and, more commonly,less than 0.5 percent by weight, based on the resin.

It is particularly useful when the foamable resin component is thehighest melting resin in the composition. In this way, the non-foamableresin powders which form the exterior surface melt before the pelletscontaining the foaming agent and can uniformly coat the interior of themold before significant foaming occurs. The resins employed for thenon-foamable powder mixture may have different melt temperatures but itis not essential for the success of the process, since other factorssuch as particle size and the molding conditions also play a role.

In one embodiment, where foamed rigid rotomolded articles havingsuperior surface quality are produced, without the use of organicperoxides, all of the resins employed have different peak melttemperatures (determined by differential scanning calorimetry).Preferably, the foamable resin is HDPE and the non-foamable powder is amixture of 75 to 97.5 percent LDPE and 2.5 to 25 percent LLDPE. Byutilizing these resins, and when all of the other specified criteria aremet, it is possible to produce foamed articles having a smooth exteriorappearance and which are substantially free of surface pores orpinholes. As used herein, the terms "essentially free" or "substantiallyfree" indicate that while some pitting may be apparent on closeexamination, these pinholes or surface pores are of a nature and numberso as not to render the surface of the molded article unacceptable,i.e., the overall appearance is not objectionable to the observer whenviewed with the naked eye. Magnification of the surface may reveal somesurface porosity, however, any surface pores present are sufficientlysmall in number and size that they do not detract from the surfaceappearance. Additionally, when using a non-foamable resin powder whichis colored, the skin layer of the rotomolded article is uniformlycolored and free of color blotches. Color blotches, or "measling" as itis also known, caused by uncolored foamable resin pellets "pushingthrough" the skin layer during the rotation is virtually eliminated withthe compositions and process of this invention.

In another useful embodiment, where compositions capable of beingrotomolded into articles which are substantially lighter in weight, anorganic peroxide is combined with the ethylene polymer and foaming agentto produce the foamable pellet component of the composition. It is evenmore advantageous when a metal-containing activator is included in thepellet component. Utilizing this approach it is not only possible tosignificantly reduce the foam density thereby reducing the weight ofrotomolded goods produced therewith, and reduce cycle times By reducingthe length of time required to produce a particular rotomolded article,i.e., the cycle time, the professor can realize significant costbenefits. By utilizing these compositions, it is also possible tobroaden the processing window. Broadening the processing window givesthe processor greater operational latitude which results in fewerarticles which need to be scrapped. By including a peroxide andactivator with the foaming agent, foam densities have been reduced by asmuch as 60% over identical formulations except for the absence ofperoxide. Similarly, cycle times have been reduced by as much as 43% andthe processing window increased by as much as 300%. All of this wasaccomplished with no sacrifice in the structural or aesthetic qualitiesof the rotomolded piece.

To achieve these improvements, up to 2.5 weight percent and, morepreferably, from 0.1 to 2.0 weight percent organic peroxide is used. Itis even more advantageous for certain formulations to employ 0.25 to 1.5weight percent organic peroxide. Any of the known organic peroxides withsufficiently high activation temperatures which will enable them to beprocessed with the foaming agent and olefin polymer to form the foamablepellet component without significant decomposition can be used. By wayof example these include: dicumyl peroxide,bis(t-butylperoxyisopropyl)benzene; di-t-butyl peroxide; t-butylcumylperoxide; 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3;2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and the like. The organicperoxides may be used as such or absorbed onto particulate materials,such as CaCO₃, clays, polyolefin resins and the like. When they areabsorbed, the weight percentage ranges specified above do not includethe particulate carrier and are especially useful organic peroxides usedeither neat or absorbed on CaCO₃. It is particularly effective whenusing a foaming agent/peroxide combination to include a metal-containingactivator at a level up to about 4 weight percent and, more preferably,form 0.01 to 2.5 weight percent, based on the weight of the olefinpolymer.

In yet another highly useful embodiment of the invention, when utilizingethylene copolymer(s) having rubbery characteristics and including anorganic peroxide and metal-containing activator in the foamablecomponent, flexible foamed rotomolded articles can be produced. Byjudicious selection of the ethylene copolymer used in either or both thefoamable and nonfoamable components and the types and amounts of foamingagent, peroxide and activator and by varying the rotomolding conditions,it is possible to produce a wide variety of flexible foamed products.For example, the texture, thickness and flexural elasticity of the skincan be varied to satisfy the requirements of diverse applications. In asimilar manner the foam density, compressive modulus and yield stresscan be varied within broad limits.

Utilizing the compositions of the inventions comprised of a foamablepellet component and a nonfoamable powder component, it is possible toproduce both rigid and flexible foamed rotomolded parts in a one-stepoperation. The process eliminates the need of introducing the foamableand non-foamable resins into the mold in two steps and also overcomesproblems associated with heretofore known one-step processes. In thepresent invention, the resin pellets containing the foaming agent andthe non-foamable resin powder are both charged to the mold at the outsetof the rotomolding operation. The pellets and powder may be addedseparately or they may be combined and the mixture charged to the moldprior to commencement of the rotomolding operation. After the pelletsand powder are introduced, the mold is closed and the rotomolding cyclecan be conducted without interruption.

For the molding process, the mold is rotated biaxially, i.e., in twodirections, utilizing conventional rotomolding equipment. No specialequipment is required to carry out the process. The powder and pelletsare moved throughout the mold and contact the interior surfaces whichenables the resins to melt and uniformly coat the interior of the mold.Interior surfaces of the mold may be treated with a suitable moldrelease agent, however, this is optional. The mold is rotated at a speedwhich permits the resin to contact the inner walls of the mold by actionof gravity. Centrifugal forces are nonexistent or minimal. Typically,heating is accomplished by placing the mold in an oven. The temperaturemust be sufficient to melt the resins and activate the foaming agent andwill generally range between about 200 to 350° C. The speed of rotationof the mold in the two directions can also be varied between widelimits. Generally, the rate of rotation will be between about 1 andabout 25 rpm. In one embodiment of the invention it has been found to beparticularly advantageous to operate at a higher rotation rate than isgenerally used in commercial practice. Whereas, ratios of 4:1 (outeraxis:inner axis) are commonly employed for commercial rotomolding,operations, with the compositions of this invention exceptional resultshave been observed when the outer axis rotation rate is from 12 to 25rpm and the inner axis rotation rate is 7 to 20 rpm.

The temperature used for the rotomolding operation will depend onvarious factors including the size of the mold, mold geometry, thicknessof the part being rotomolded, the foaming agent used and the resinsemployed. Similarly, the length of time required to rotomold the articlewill depend on these factors and the temperature. As a result, time andtemperature will vary within wide limits. For example, to mold a tankhaving a part thickness of approximately 0.5 inch using one of the apreferred compositions of the invention wherein the foamable resin isHDPE and the non-foamable powder is a mixture of LDPE and LLDPE, atemperature in the range of about 225° C. to 300° C. is preferred for atime of 20 to 50 minutes.

These and other features of the invention are illustrated in more detailin the examples which follow. All parts and percentages in the examplesare on a weight basis unless otherwise indicated.

EXAMPLE 1

A rotomolding composition comprising 40 percent foamable resin pelletsand 60 percent non-foamable resin powder was prepared in accordance withthe invention. The foamable resin was HDPE (MI 5.5; density 0.961)containing 0.6 percent azodicarbonamide obtained by pre-compounding theHDPE and foaming agent in an extruder and pelletizing. The non-foamableresin powder was a mixture of 90 percent LDPE (MI 0.25; density 0.918)and 10 percent LLDPE (MI 3.5; density 0.939) powders of differentparticle size obtained by grinding. Particle size distributions of thetwo powders (determined by shaking a known amount of powder through astack of screens of different mesh size and measuring the amount ofmaterial retained) were as follows:

    ______________________________________                                                    LDPE Powder                                                                              LLDPE Powder                                           ______________________________________                                        >590 microns    0%           0%                                               500-590 microns                                                                              8.6%          0%                                               420-449 microns                                                                             44.3%          0%                                               250-419 microns                                                                             29.4%         1.8%                                              180-249 microns                                                                              9.7%        44.9%                                              150-179 microns                                                                              3.6%        19.1%                                              <150 microns   4.4%        34.2%                                              Mean Particle Size                                                                           370          185                                               ______________________________________                                    

To facilitate visual examination and comparison of rotomolded partsproduced from the rotomolding composition, 1.5 percent inorganic redpigment (DC-22552 RED from Teknor Color Company) was melt blended withthe LLDPE resin before grinding.

Approximately 71/2 pounds of the powder/pellet mixture was introducedinto an aluminum tank mold measuring 13×20×3 inches vented with aTeflon® tube containing a steel wool plug. The rotomolding operation wascarried out using a single arm shuttle rotomolding machine and thefollowing conditions:

Temperature 250° C.

Time 45 minutes

Rotation Rate:

Outer Axis 19 rpm

Inner Axis 15 rpm

Cooling Cycle:

10 minutes air (with fan)

10 minutes water spray

10 minutes air (with fan)

The rotomolded tank had good rigidity and mechanical strength. Theexterior skin was smooth and substantially free of surface pitting.Furthermore, the skin layer was uniformly colored with no indication ofthe uncolored interior foam layer pushing through the surface layer.Sectioning the tank revealed a sharp boundary between the foam and skinlayers and uniform thickness of the layers throughout the entire part.Furthermore, the foam layer had uniform cell structure with a smooth,continuous interior surface.

COMPARISON A

To demonstrate the necessity of using a mixture of powders, when Example1 is repeated except that the non-foamable resin powder is comprisedsolely of either (a) 100 percent of the fractional melt index LDPEpowder or (b) 100 percent of the 3.5 MI LLDPE powder, it is not possibleto produce articles having good layer distinction and a non-pittedexterior surface. In the first case (a), where the higher MI, smallerparticle size LLDPE powder is omitted, the skin layer of the tank isheavily pitted and unacceptable. In the second case (b), where thefractional melt index, larger particle size LDPE powder is omitted, thesurface is smooth but "measled," i.e., uncolored or lightly coloredblotches are evident on the surface as a result of uncolored foamingresin mixing with the colored skin layer.

COMPARISON B

To demonstrate the need for the major, larger particle size powdercomponent to have a fractional melt index, Example 1 was repeated exceptthat different LDPE resins were employed in the makeup of the powdermixture. In the first instance, the LDPE was a 2.1 MI, 0.922 densityresin and in the second instance, the LDPE had an MI of 3.7 and densityof 0.923. The powder particle size distribution was essentially the sameas for the fractional melt index LDPE of Example 1. While rotomoldedtanks produced using the two comparative resins were smooth andessentially free of pitting, numerous light colored areas or blotcheswhere the uncolored foamed resin mixed with the colored skin layer wereapparent. In the case of the 2.1 MI LDPE, there were 0.15 blemishes persquare inch of surface whereas 0.21 blemishes per square inch wererecorded with the 3.7 MI LDPE. Furthermore, the thickness of the foamlayer was not uniform in either case and, with the 3.7 MI resin, therewas incomplete foam coverage.

EXAMPLE 2

To further illustrate the invention and the ability to rotomoldfoam-filled parts having a smooth, uniformly colored, pit-free exteriorskin, a composition comprised of 50 percent pellets and 50 percentpowder was prepared and evaluated. For this experiment the HDPE pelletscontained 1 5 percent azodicarbonamide and the HDPE had an MI of 4.0 anddensity of 0.946. The powder was the same LDPE/LLDPE mixture as used forExample 1. Rotomolding was conducted as previously described except thatthe venting system was modified and the cooling cycle was as follows:

20 minutes air

20 minutes air (with fan)

10 minutes water spray

10 minutes air (with fan)

A molded tank produced using the above-described composition had asmooth skin layer and the interior was completely foamed. The surfacewas substantially free of pinholes and no color variation was observed.

EXAMPLE 3

To further demonstrate the need for the smaller particle size powdercomponent and the effect of particle size, a series of rotomoldingcompositions were prepared containing 50 percent HDPE pellets and 50percent of a mixture of LDPE and LLDPE powders. The HDPE component hadan MI of 8 and density of 0.963 and the pellets contained 0.6 percentazodicarbonamide. The LDPE (MI 0.25; density 0.922) powder used for allthe products had essentially the same particle size distribution as inExample 1; however, the LLDPE (MI 3.5; density 0.939) particle sizedistribution was varied. Details of the compositions and particle sizedistributions of the various LLDPE powders used are set forth in thetable which follows. Compositions 3(a) and 3(b) are products of theinvention whereas compositions C, D, E and F are provided forcomparative purposes.

Each composition was used to rotomold a foamed hexagonal article and thesurface porosity evaluated. For the rotomolding operation, 2.4 lbs ofthe composition was charged to a steel hexagonal mold measuring 12inches across and 5 inches deep. The mold was heated at 350° C. for 25minutes while rotating at a rate of 9 rpm (inner axis) and 16 rpm (outeraxis). The cooling cycle consisted of 10 minutes water spray and 10minutes air cooling with a fan. The number of surface pores or pinholesin a 1.2 cm×1 cm area was measured using the Global Lab Image Analysis"Particle" Tool/Function. The image was generated by a camera attachedto a stereomicroscope (15× magnification). The measurement area was heldconstant by maintaining the scanning region borders at their outerlimits. Two areas at the approximate same location on each of thearticles were examined and the number of surface pores determined byaveraging eight readings (four readings in each area). Results arereported in the table. The data clearly show the significant andunexpected reduction in the number of pinholes with the compositions ofthe invention when the minor powder component has a particle size lessthan 250 microns. As the particle size of the minor (LLDPE) component inthe powder mixture is increased, there is a marked increase in thenumber of surface pits on the rotomolded part.

    __________________________________________________________________________    PRODUCT     3(a)                                                                             3(b)                                                                             C    D    E    F                                            __________________________________________________________________________    Powder Composition:                                                           % LDPE      90 85 90   85   90   85                                           % LLDPE     10 15 10   15   10   15                                           LLDPE Particle Size                                                           (microns)   <250                                                                             <250                                                                             250-297                                                                            250-297                                                                            420-590                                                                            420-590                                      Average no. of surface pits                                                               10 6  121  67   188  176                                          __________________________________________________________________________

EXAMPLE 4

Example 3(a) was repeated except that a LDPE (MI 1.8; density 0.923)containing 0.6 percent azodicarbonamide was employed as the foamableresin. Rotomolded hexagonal articles produced therefrom had a smooth,uniformly colored skin layer.

EXAMPLE 5

Repeating Example 4 but substituting a fractional melt index HDPE powderfor the LDPE powder also save foamed rotomolded articles with exteriorskins which were smooth and substantially tree of surface pitting.

EXAMPLE 6

To demonstrate the ability to include additives in the foamable resin, arotomolding composition was prepared by combining the following:

    ______________________________________                                        3.80 parts                                                                              HDPE pellets (MI 4.3, density 0.953) containing                               0.5 wt. % zinc oxide and 1.8 wt. % azodicarbonamide                 0.41 parts                                                                              LLDPE powder of Example 1                                           3.97 parts                                                                              LDPE powder of Example 1                                            ______________________________________                                    

The above composition was used to rotomold a tank using the same mold,resin charge, temperature and rotation rate employed for Example 1.Tanks of comparable quality were produced using shorter cycle timeswhich results in significant cost benefits. By the addition of ZnO tothe foamable resin pellets, it was possible to reduce the cycle time byapproximately 15 percent without sacrificing foam or surface qualitiesof the rotomolded part.

EXAMPLE 7

Foamable pellets containing 2 wt. % azodicarbonamide were preparedutilizing an HDPE resin having a MI of 4 and density of 0.946. Thesepellets were combined with the LDPE/LLDPE powder mixture of Example 1 ata ratio of 48:52 (pellet:powder) and the compositions rotomolded at 316°C. Tanks of good quality, i.e., having good rigidity and surfaceappearance with a uniformly foamed interior, were obtained. The oventime required to rotomold the tanks. i.e, cycle time, was 20 minutes.The foam had a density of approximately 8 lbs./ft.³

EXAMPLE 8

To demonstrate the ability to reduce the cycle time by incorporating ametal-containing activator, Example 7 was repeated except 0.2 wt. % zincoxide was compounded into the foamable HDPE pellets along with theazodicarbonamide. By including the activator, the time required toproduce rotomolded tanks was reduced to approximately 18 minutes withoutcompromising the quality of the tanks. Foam density was approximately 8lbs/ft.³

EXAMPLE 9

The ability to alter the foam characteristics, specifically, to reducethe foam density, by further varying the compositions of the inventionis illustrated by this example. For this experiment, rigid foamedarticles were rotomolded utilizing foamable LDPE pellets. The LDPE had aMI of 7 and density of 0.918 and was compounded with 2%azodicarbonamide, 2% zinc oxide and 0.5% LUPERCO XL (45-48%2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3) The pellets were combinedwith the LDPE/LLDPE powder mixture at a ratio of 48:52 (pellet:powder)and the resulting compositions charged to the tank mold In androtomolded at 316° C. to produce rigid, foam-filled tanks. The cycletime was 18 minutes. Tanks of comparable surface quality to thoseproduced in Examples 7 and 8 and having a uniform, line cell size foamedinterior; however, the foam density was advantageously lowered toapproximately 7 lbs/ft³. Foams produced in this manner have compressivemodulii (ASTM C 165) in the range of 160-550 psi. In addition to thedesirable reduction in the weight of the article produced using thefoamable composition containing the organic peroxide, there was asignificant broadening of the processing window. The processing windowis the difference between the maximum and minimum oven times in which anacceptable part can be produced. Under the conditions employed, theformulation of this example had a processing window of 4 minutes, i.e.,acceptable rotomold parts could be obtained using cycle times from 18 to22 minutes; whereas the compositions of Examples 6 and 7 had processingwindows of 1 minute (21-20 minutes) and 2 minutes (20-18 minutes),respectively.

EXAMPLES 10 AND 11

To demonstrate the ability to reduce the foam density to an even greaterextent and further improve, i.e, reduce, cycle times, two experimentswere conducted following the procedure of Example 9 utilizing LDPEpellets containing a foaming agent, metal-containing activator andorganic peroxide. The foamable pellets were comprised as follows:

Example 10: 93.5% LDPE

5.0% azodicarbonamide

0.5% zinc oxide

1.0% LUPERCO XL

Example 11: 93.5% LDPE

5.0% azodicarbonamide

0.5% zinc oxide

5% LUPERCO XL

These pellets were combined at the same ratio with the LDPE/LLDPE powderand rotomolded at 316° C. to produce semi-rigid tanks having smoothsurfaces substantially free of surface pitting. Compared to therotomolding operation of Example 9, foam densities were reduced byapproximately 40% and cycle times reduced by approximately 10%. Forexample, the foam produced using the composition of Example 10 had adensity of approximately 4 lbs/ft.³ and the cycle time was 16 minutes.

EXAMPLE 12

Flexible foam-filled, light weight articles having a rubbery copolymerskin were produced using a composition of 1.5 parts ethylene-vinylacetate (19% VA) copolymer powder (35 mesh) and 0.75 part foamablepellets comprised of 87.5% LDPE, 10% azodicarbonamide, 1% zinc oxide and1.5% LUPERCO XL. The composition was rotomolded at approximately 255° C.using a hexagonal mold as described in Example 3. Cycle time used was 15minutes. The foam had a density of approximately 2 lbs./ft.³ andcompressive modulus less than 100 psi. This procedure can also beutilized to produce articles having a more rigid skin covering a softfoam by substituting LDPE or a mixture of LDPE/LLDPE, such as used inExample 1, for all or a portion of the EVA in the above-identifiedpowder component.

EXAMPLE 13

The versatility of the present invention and the ability to produce evenlighter weight flexible, foam-filled articles is illustrated by thisexample wherein, following the procedure of Example 12, LDPE pellets andEVA powder were combined and rotomolded in a hexagonal mold. The EVApowder was the same as utilized in Example 12, however, the foamablepellets comprised 82% LDPE, 15% azodicarbonamide, 1.5% zinc oxide and1.5% LUPERCO XL. Rotomolded articles produced utilizing this formulationhad a foam density of approximately 1.5 lbs/ft.³ and compressive modulusof approximately 8 psi. Among the many uses of flexible, foam-filledarticles of this type include use as cushioning devices and the like.Even softer foamed articles can be produced by reducing or evencompletely replacing the LDPE in the foamable pellets with a copolymerhaving more rubbery characteristics.

EXAMPLE 14

This example further demonstrates the ability to reduce cycle times. Theexperiment was carried out utilizing an LDPE/LLDPE powder mixture withLLDPE pellets containing 3.5% oxybis(benzenesulfonylhydrazide) and 0.5%LUPERCO XL. The LLDPE utilized for the preparation of the foamablepellets had a melt index of 3.5 and density of 0.939. When thecompositions were rotomolded at 293° C. using Teflon hexagonal tankmolds, cycle times of approximately 12 minutes were achieved and theresulting foamed tanks had good surface quality and rigidity. Under thesame molding conditions, the composition of Example 8 required a cycletime of 30 minutes. The foam had a density of approximately 8-9lbs/ft.³. Foam densities of the compositions can be reduced by utilizinga higher melt index, e.g., 6 MI, LLDPE resin.

I claim:
 1. A process for rotationally molding articles having a foamedinterior and a non-foamed exterior skin which comprises introducing arotomolding composition comprising polymers to a mold and heating themold and its contents to a temperature above the melt temperature of thepolymers and above decomposition temperatures of a chemical foamingagent and an organic peroxide while biaxially rotating, said rotomoldingcomposition comprising:(a) 25 to 75 wt. %, based on the weight of thetotal composition, ethylene polymer pellets ranging in size from about1/16 inch to about 3/16 inch in diameter and containing 0.25 to 7.5 wt.% chemical foaming agent, based on the weight of the ethylene polymerand 0.1 to 2.0 wt. % organic peroxide, based on the weight of theethylene polymer, said ethylene polymer having a melt index from 0.25g/10 mins to 25 g/10 mins; and (b) 75 to 25 wt. %, based on the weightof the total composition, ethylene polymer powder containing:(i) a majorproportion of fractional melt index ethylene polymer powder wherein 80percent or more of the powder particles are greater than 250 microns insize and (ii) a minor proportion of ethylene polymer powder wherein 80percent or more of the powder particles are less than 250 microns insize and the ethylene polymer has a melt index greater than 1 g/10mins., and with the proviso that the mean particle size of (b)(i) and(b)(ii) differ by at least 100 microns.
 2. The process of claim 1wherein (a) of the rotomolding composition comprises from 30 to 60weight percent of the total composition and (b) comprises 40 to 70weight percent of the total composition.
 3. The process of claim 2wherein the foaming agent of the rotomolding composition is selectedfrom the group consisting of azodicarbonamide, modifiedazodicarbonamide, oxybis(benzenesulfonylhydrazide), sodium bicarbonateand sodium bicarbonate/citric acid and the organic peroxide is selectedfrom the group consisting of dicumyl peroxide,bis(t-butylperoxyisopropyl)benzene, di-t-butyl peroxide, t-butylcumylperoxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3 and2,5-dimethyl-2,5-di(t-butylperoxy)hexane.
 4. The process of claim 3wherein (a) of the rotomolding composition also contains up to 4 weightpercent metal-containing activator, based on the weight of the ethylenepolymer, selected from the group consisting of zinc oxide or zincstearate.
 5. The process of claim 4 wherein the foaming agent of therotomolding composition is azodicarbonamide, modified azodicarbonamideor oxybis(benzenesulfonyl-hydrazide) present in an amount from 0.5 to 5wt. %, based on the weight of the ethylene polymer.
 6. The process ofclaim 5 wherein (a) of the rotomolding composition is a high densityethylene homopolymer or copolymer of ethylene and C₃₋₈ α-olefin having amelt index from 1 to 10 g/10 mins and density from 0.941 to 0.970 g/cm³.7. The process of claim 6 wherein (a) of the rotomolding composition isa copolymer of ethylene and butene-1 or hexene-1 having a density of0.941 to 0.959 g/cm³.
 8. The process of claim 5 wherein (b)(i) of therotomolding composition constitutes from 75 to 97.5 percent of the totalpowder mixture and (b)(ii) constitutes from 2.5 to 25 percent of thetotal powder mixture.
 9. The process of claim 8 wherein (b)(i) of therotomolding composition has a particle size of from 250 to 1000 micronsand is a low density ethylene homopolymer having a melt index from 0.1to 0.8 g/10 mins and density from 0.915 to 0.930 g/cm³.
 10. The processof claim 7 wherein (b)(ii) of the rotomolding composition has a particlesize from 10 to 250 microns and is a low density copolymer of ethylenewith butene-1, hexene-1 or octene-1 having a melt index from 1 to 10g/10 mins.
 11. The process of claim 10 wherein (b)(ii) of therotomolding composition is a linear LDPE having a particle size from 20microns to 225 microns, density from 0.915 to 0.940 g/cm³, and meltindex from 2 to 7 g/10 mins.
 12. The process of claim 7 wherein theorganic peroxide of the rotomolding composition is2,5-dimethyl-2,5-di(t-butylperoxy) hexyne-3 or2,5-dimethyl-2,5-di(t-butylperoxy) hexane present in an amount form 0.25to 1.5 wt. %, based on the weight of the ethylene polymer and optionallyadsorbed onto calcium carbonate or clay.
 13. A process for rotationallymolding flexible foamed-filled articles which comprises introducing arotomolding composition comprising polymers to a mold and heating themold and its contents to a temperature above the melt temperature of thepolymers and above decomposition temperatures of a chemical foamingagent and an organic peroxide while biaxially rotating, said rotomoldingcompositions comprising:(a) 25 to 75 wt. %, based on the weight of thetotal composition, ethylene polymer pellets ranging in size form 1/16inch to about 3/16 inch in diameter and containing 0.25 to 20 wt. %chemical foaming agent, based on the weight of the ethylene polymer, 0.1to 2.5 wt. % organic peroxide, based on the weight of the ethylenepolymer, and 0.1 to 4 wt. % metal-containing activator, based on theweight of the ethylene polymer; said ethylene polymer having a meltindex from 0.25 g/10 mins to 25 g/10 mins, and (b) 75 to 25 wt. %, basedon the weight of the total composition, of an ethylene copolymer powderwherein the ethylene copolymer has less than 30% crystallinity asdetermined by differential scanning calorimetry and is selected from thegroup consisting of ethylene-vinyl acetate copolymers, ethylene-alkylacrylate copolymers and ethylene-α-olefin copolymers.
 14. The process ofclaim 13 wherein the foaming agent of the rotomolding composition isselected from the group consisting of azodicarbonamide, modifiedazodicarbonamide, oxybis(benzenesulfonylhydrazide), sodium bicarbonateand sodium bicarbonate/citric acid and the organic peroxide is selectedfrom the group consisting of dicumyl peroxide,bis(t-butylperoxyisopropyl)benzene, di-t-butyl peroxide, t-butylcumylperoxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3 and2,5-dimethyl-2,5-di(t-butylperoxy)hexane.
 15. The process of claim 14wherein the ethylene copolymer (b) is an ethylene-vinyl acetatecopolymer containing from 8 to 50% vinyl acetate, an ethylene-alkylacrylate copolymer containing 1 to 50% ethyl acrylate or n-butylacrylate and an ethylene-α-olefin copolymer containing 20 to 80% of anα-olefin having from 3 to 8 carbon atoms.
 16. The process of claim 15wherein (a) of the rotomolding composition comprises from 30 to 60 wt. %of the total composition and (b) comprises from 40 to 70 wt. % of thetotal composition.
 17. The process of claim 16 wherein the ethylenepolymer is LDPE.
 18. The process of claim 17 wherein the ethylenecopolymer has less than 20% crystallinity as determined by differentialscanning calorimetry.
 19. The process of claim 17 wherein themetal-containing activator of the rotomolding composition is zinc oxideor zinc stearate present in an amount from 0.01 to 2.5 wt. %, based onthe weight of the ethylene polymer.
 20. The process of claim 17 whereinthe organic peroxide of the rotomolding composition is2,5-dimethyl-2,5-di(t-butylperoxy) hexyne-3 or2,5-dimethyl-2,5-di(t-butylperoxy) hexane present in an amount form 0.25to 1.5 wt. %, based on the weight of the ethylene polymer.
 21. Theprocess of claim 17 wherein the foaming agent of the rotomoldingcomposition is azodicarbonamide, modified azodicarbonamide oroxybis(benzenesulfonyl-hydrazide) present in an amount from 0.5 to 5 wt.%, based on thee weight of the ethylene polymer.
 22. The process ofclaim 17 wherein the ethylene copolymer of the rotomolding compositionis an ethylene-vinyl acetate copolymer containing from 10 to 45% vinylacetate.